Conventionally, there have been proposed a fluid control apparatus of this type which is shown in FIG. 9 as comprising a hydraulic fluid actuator 10, a piston rod 10a, and a piston 10b securely connected with the piston rod 10a and axially movably received in the hydraulic fluid actuator 10 to define first and second chambers 11 and 12. The piston rod 10a is axially movable between two axial positions consisting of an extension position where the hydraulic fluid is supplied to the first chamber 11 and discharged from the second chamber 12, and a retraction position where the hydraulic fluid is supplied to the second chamber 12 and discharged from the first chamber 11.
The fluid control apparatus further comprises a housing 13, a servo valve 14 securely mounted on the housing 13 and having supply and return ports 14a and 14b and first and second pressure control ports 14c and 14d, a swivel joint member 15 rotatably received in the housing 13 and having supply and return ports 15a and 15b to have the hydraulic fluid supplied to the supply port 14a of the servo valve 14 and discharged from the return port 14b of the servo valve 14, respectively, a filter 16 provided in the fluid passageways between the swivel joint member 15 and the servo valve 14 to filtrate the hydraulic fluid passing therethrough. The servo valve 14 is operated by two external signals to selectively bring about first and second flow conditions of the hydraulic fluid to the hydraulic fluid actuator 10. The first flow condition is accomplished by having the first pressure control port 14c brought into fluid communication with one of the first and second chambers 11 and 12 of the hydraulic fluid actuator 10. The second flow condition, on the other hand, is accomplished by having the second pressure control port 14d brought into fluid communication with the other of the first and second chambers 11 and 12 of the hydraulic fluid actuator 10.
The fluid control apparatus further comprises a check valve 17 provided in the fluid passageway between the second pressure control port 14d and the second chamber 12 of the hydraulic fluid actuator 10 to serve as being opened by the pressure in the hydraulic fluid from the supply port 15a of the swivel joint member 15 and being closed by the spring, not shown in the drawings, of the check valve 17. The check valve 17 is operated to be closed to prevent the piston rod 10a of the hydraulic fluid actuator 10 from being extended when the fluid control apparatus falls into an abnormal condition having the pressure of the hydraulic fluid drastically decline resulting from some reasons. The opening operation of the check valve 17 can be performed by manually operating a manual relief valve 18 operatively connected to the check valve 17.
In order to allow the check valve 17 to be closed to prevent the piston rod 10a of the hydraulic fluid actuator 10 from being extended over a predetermined limit when the fluid control apparatus falls into such an abnormal condition, the fluid control apparatus is required to be of a coaxial type valve which comprises a spool valve, and a sleeve having the spool valve coaxially movably received therein and formed with ports through which the hydraulic fluid passes to the spool valve. This means that the conventional fluid control apparatus herein described encounters some problems that it is not only complex in construction but also increased in weight, size and costly. Moreover, the conventional fluid control apparatus tends to deteriorate in reliability.
It is therefore an object of the present invention to provide a fluid control apparatus which is simple in construction and has a high reliability.
It is another object of the present invention to provide a fluid control apparatus which is light, small in size and relatively inexpensive.